Field of the Invention
The present invention generally relates to the design and control of internal combustion engines that can operate on two different fuels and the relative power of the engines at the different fuels.
Description of the Related Art
Internal combustion engines operate on the principle of igniting a mixture of air and gasoline (or other fuel) inside a cylinder to cause combustion within the cylinder where resulting released energy is converted to mechanical energy through the use of a piston inside the cylinder driving a crankshaft. A fuel intake assembly, such as gasoline fuel injectors, is used to inject gasoline into cylinders or intake system of the engine. Internal combustion engines are typically naturally aspirated meaning that air is drawn from the environment at atmospheric pressure. As a result of the combustion of the air fuel mixture within a cylinder of the engine, different types of unwanted toxic and pollutant gases are created in the cylinder and pass through an exhaust system to a device commonly referred to as a catalytic converter.
Typically, internal combustion engines (especially those used for automobiles) use gasoline (or diesel) as a fuel which when burned in an internal combustion engine generates exhaust gases some of which are pollutants and/or toxic. Other less polluting carbon based fuels or even non-carbon based fuels can be used, but many of these fuels when mixed with air don't have as nearly the same energy content (i.e., ‘mixture calorific value’) for nearly the power output as when burning gasoline (i.e., gasoline mixed with air). It is desirable, however, to use some of these fuels because they can be used in what is referred to as a “lean mode” operation where virtually no toxic or polluting gases are generated from the combustion process. Operation in the lean mode refers to the air/fuel ratio with which the engine is being operated. In particular, the ratio of the amount of air and fuel in a combustion chamber of the engine will determine whether the engine is being operated in the lean mode or rich mode. For an ideal combustion the amount of air and fuel used for combustion in a chamber of an engine is such that there is no residual oxygen or fuel remaining in the chamber after combustion, the particular air fuel ratio is referred to as the stoichiometric air fuel ratio. The actual air fuel ratio, however, may not be at stoichiometric at all times. A ratio of the actual air fuel ratio to the stoichiometric air fuel ratio is referred to as λ. The variable λ is thus defined mathematically as:
                    λ        =                                            (                                                mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  air                                                  mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  fuel                                            )                        ⁢            actual                                              (                                                mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  air                                                  mass                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  fuel                                            )                        ⁢            stoichiometric                                              (        1        )            
When λ=1 the engine is operated at the stoichiometric air fuel ratio because the actual air fuel ratio is equal to the stoichiometric air fuel ratio as can be seen from equation (1) above. For value of λ<1, the engine is said to be in the rich mode. For λ>1, the engine is said to be in the lean mode. As can be seen from equation (1) above, in the lean mode more air is used in the combustion than in the stoichiometric mode. As a result operation in the lean mode although more desirable because such a mode generates a lesser amount of harmful and/or pollutant gases, the loss of engine output power is exacerbated. Lean mode operation also results in a relatively high efficient operation of the engine because of the relatively low fuel consumption. This relatively high efficiency is often not realized because conventional gasoline engines typically do no run in lean mode.
Efficiency refers to fuel consumption for a defined engine output power. The more efficiently an engine is operating, the lower the fuel consumption for a specific engine output power. Increasing the efficiency of an engine results in lowering the fuel consumption of the engine. In other words, an engine can generate a specific output power by consuming a certain amount of fuel, but the same engine when run efficiently can generate the same amount of output power while consuming less fuel. One way to increase the efficiency of an internal combustion engine is to run the engine in lean mode.